1. Field
The present invention relates generally to data communication, and more specifically to techniques for controlling the transmit power of a data transmission that uses multiple formats (e.g., rates, transport formats) as supported by a communication system using power control (e.g., W-CDMA).
2. Background
In a wireless communication system, a user with a terminal (e.g., a cellular phone) communicates with another user via transmissions on the downlink and uplink through one or more base stations. The downlink (i.e., forward link) refers to transmission from the base station to the terminal, and the uplink (i.e., reverse link) refers to transmission from the terminal to the base station. The downlink and uplink are typically allocated different frequencies.
In a Code Division Multiple Access (CDMA) system, the total transmit power available for a base station is typically indicative of the total downlink capacity for that base station since data may be concurrently transmitted to a number of terminals over the same frequency band. A portion of the total available transmit power is allocated to each active terminal such that the aggregate transmit power for all active terminals is less than or equal to the total available transmit power.
To maximize the downlink capacity, a power control mechanism is typically used to minimize power consumption and interference while maintaining the desired level of performance. Conventionally, this power control mechanism is implemented with two power control loops. The first power control loop (often referred to as an “inner” power control loop, or simply, the inner loop) adjusts the transmit power to each terminal such that the signal quality of the transmission received at the terminal (e.g., as measured by a signal-to-noise-plus-interference ratio (SNIR)) is maintained at a particular target SNIR. This target SNIR is often referred to as the power control setpoint (or simply, the setpoint). The second power control loop (often referred to as an “outer” power control loop, or simply, the outer loop) adjusts the target SNIR such that the desired level of performance (e.g., as measured by a particular target block error rate (BLER), frame error rate (FER), or bit error rate (BER)) is maintained. By minimizing the amount of transmit power while maintaining the target BLER, increased system capacity and reduced delays in serving users can be achieved.
A W-CDMA system supports data transmission on one or more transport channels, and one or more transport formats may be used for each transport channel. Each transport format defines various processing parameters such as the transmission time interval (TTI) over which the transport format applies, the size of each transport block of data, the number of transport blocks within each TTI, the coding scheme to be used for the TTI, and so on. The use of multiple transport formats allows different types or rates of data to be transmitted over a single transport channel.
The W-CDMA standard currently permits one target BLER to be specified by the base station for each transport channel, regardless of the number of transport formats that may be selected for use for the transport channel. Each transport format may be associated with a different code block length, which may in turn require a different target SNIR to achieve the target BLER. (For W-CDMA, the code block length is determined by the transport block size, which is specified by the transport format.) In W-CDMA, one or more transport channels are multiplexed together in a single physical channel, whose transmit power is adjusted through power control. Using the conventional power control mechanism, the inner power control loop would adjust the target SNIR based on the received transport blocks to achieve the target BLER or better for each transport channel.
Since different transport formats may require different target SNIRs to achieve the target BLER, the average transmit power for the physical channel may fluctuate depending on the specific sequence of transport formats selected for use in the constituent transport channel(s) (i.e., the relative frequency of the transport formats and their ordering). And since the outer and inner loops take some amount of time to converge, each time the transport format is changed, a transient occurs until the loops converge on the target SNIR for the new transport format. During this transient time, the actual BLER may be much greater or less than the target BLER, which would then result in degraded performance and lower system capacity.
There is therefore a need in the art for an improved power control mechanism for a (e.g., W-CDMA) communication system capable of transmitting data on one or more transport channels using multiple transport formats.